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ANGAS BREMER PWA GROUNDWATER LEVEL AND SALINITY STATUS REPORT

2009–10

Angas Bremer PWA Groundwater Status Report 2009–10

DEPARTMENT OF ENVIRONMENT, WATER AND NATURAL RESOURCES | 1 |

Science, Monitoring and Information Division

Department for Environment, Water and Natural Resources

25 Grenfell Street, Adelaide

GPO Box 2834, Adelaide SA 5001

Telephone National (08) 8463 6946

International +61 8 8463 6946

Fax National (08) 8463 6999

International +61 8 8463 6999

Website www.waterforgood.sa.gov.au

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© Crown in right of the State of South Australia, through the Department of Environment, Water and Natural Resources

2012

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Environment, Water and Natural Resources, GPO Box 2834, Adelaide SA 5001.

ISBN 978-1-921923-45-6

This document is available online at http://www.waterconnect.sa.gov.au/GSR



2009–10 SUMMARY

The Angas Bremer Prescribed Wells Area (PWA) is located on the western side of Lake Alexandrina approximately 60 km south-east of Adelaide. It is a local-scale resource for which groundwater is prescribed under South Australia’s Natural Resources Management Act 2004. A Water Allocation Plan provides for sustainable management of the PWA’s groundwater resources.

Groundwater extractions occur from the confined Murray Group Limestone aquifer, which can be up to 100 m thick. Metered groundwater extractions for licensed purposes in the Angas Bremer PWA totalled 5334 ML in the 2009–10 water-use year, which represents a 17% decrease from the previous water-use year.

The construction of two pipelines from the River Murray during the recent drought has increased the reliability and the volumes of watercourse water available for managed aquifer recharge. Managed aquifer recharge during 2009–10 was 4803 ML, which represents a significant increase of 123% when compared to the previous year (being 2152 ML).

The recovery of Lake Alexandrina and the construction of two new pipelines from the River Murray have reduced demand on groundwater (from the high extraction levels experienced since the 2006 drought) and facilitated an increase in managed aquifer recharge. This has led to a rise in groundwater levels in the confined limestone aquifer during 2009–10. Groundwater levels have also risen in the overlying Quaternary aquifer in response to increased recharge from rainfall and streamflow.

Widespread decreases in groundwater salinity in the confined limestone aquifer were observed during 2009–10 following the significant rises observed due to the increased extractions since the 2006 drought. These decreases are due to managed aquifer recharge using lower-salinity water and reduced downward leakage from the overlying saline Quaternary aquifer in response to reduced drawdown in the confined limestone aquifer brought about by lower extractions.



ASSESSMENT OF STATUS The Angas Bremer PWA has been assigned a green status of “No adverse trends, indicating a stable or improving situation” for the 2009–10 water-use year. This status is supported by:

a recovery in groundwater levels over most of the PWA

decreases in salinity levels in most areas of the confined limestone aquifer.

The positive trends, whilst occurring after a period of previously adverse trends resulting from high extraction during the recent (2006) drought period, are expected to continue if the volumes of managed aquifer recharge are maintained or increased and if extraction levels fall to pre-drought levels (below 2000 ML/y).

Continual extractions at levels during the drought (approximately 6500 ML/y) have been demonstrated to be unsustainable in the medium term.

2009–10 STATUS

No adverse trends, indicating a stable or improving situation

Trends are either stable (no significant change), or improving (i.e. decreasing salinity or rising water levels).

Adverse trends indicating low risk to the resource in the medium term

Observed adverse trends are gradual and if continued, will not lead to a change in the current beneficial uses of the groundwater resource for at least 15 years. Beneficial uses may be drinking water, irrigation or stock watering.

Adverse trends indicating high risk to the resource eventuating in the short to medium term

Observed adverse trends are significant and if continued, will lead to a change in the current beneficial uses of the groundwater resource in about 10 years.

Degradation of the resource compromising present use within the short term

Trends indicate degradation of the resource is occurring, or will occur within 5 years. Degradation will result in a change in the beneficial use (i.e. no longer suitable for drinking or irrigation purposes) and may take the form of increasing groundwater salinities, or a fall in the groundwater levels such that extractions from the aquifer may not be possible.



BACKGROUND

The Angas Bremer Prescribed Wells Area (PWA) is located on the western side of Lake Alexandrina, approximately 60 km south-east of Adelaide (Fig. 1). It is a local-scale resource for which groundwater is prescribed under South Australia’s Natural Resources Management Act 2004. A Water Allocation Plan provides for sustainable management of the PWA’s groundwater resource.

Figure 1. Location of Angas Bremer PWA



HYDROGEOLOGY The Angas Bremer PWA is underlain by sediments of the western margin of the Murray Basin where Quaternary and Tertiary sediments are deposited over the Cambrian basement rocks. The hydrogeology of the main aquifers is discussed below.

QUATERNARY AQUIFER This aquifer system has developed within a 10–20 m thick sequence of Quaternary sediments that consist mainly of clays, silt, sands and occasional gravels. The Quaternary aquifer is generally highly saline with low yields and has limited use. Good-quality groundwater is only found within narrow zones of rapid recharge, which occur along the Angas and Bremer Rivers during flooding events. This aquifer is mostly unconfined.

CONFINED LIMESTONE AQUIFER

The confined Murray Group Limestone aquifer is up to 100 m thick and varies in composition from soft clayey limestone, hard sandy limestone to soft bryozoal layers. Irrigation supplies are generally obtained from the fossiliferous limestone member, which can be cavernous in some areas. Well yields vary from about 5 L/s in the north to over 15 L/s to the south, with occasional yields of up to 40 L/s.

The low-permeability aquitard comprising the Ettrick Formation (grey-green glauconitic marl) separates the overlying limestone aquifer from the underlying Renmark Group confined aquifer.

RENMARK GROUP AQUIFER

The Renmark Group aquifer consists of interbedded sands, silt and carbonaceous clay. It is generally thin and discontinuous over the Angas Bremer PWA and is not widely used due to the availability of much larger supplies in the overlying Murray Group Limestone aquifer.

A schematic cross section of the Angas Bremer PWA is displayed in Figure 2.



Figure 2. Geological cross section of the Angas Bremer PWA

For further information regarding the hydrogeology and previous status of the PWA, see: http://www.waterconnect.sa.gov.au/BusinessUnits/InformationUnit/Technical%20Publications/ki_dwlbc_report_2007_27.pdf A summary of the hydrogeology is presented in Table 1

MURRAY GROUP LIMESTONE AQUIFER

QUATERNARY AQUIFER

RENMARK GROUP AQUIFER

BASEMENT

-100

-50

50

0

0 1 2 3

http://www.waterconnect.sa.gov.au/BusinessUnits/InformationUnit/Technical%20Publications/ki_dwlbc_report_2007_27.pdf

http://www.waterconnect.sa.gov.au/BusinessUnits/InformationUnit/Technical%20Publications/ki_dwlbc_report_2007_27.pdf



Table 1. Hydrogeology of the Angas Bremer PWA

AGE STRATIGRAPHY HYDROGEOLOGY

Unit Lithology Unit Description Q

uat

ern

ary

Various Quaternary sediments

Alluvial and lacustrine clays, sands, silts with occasional gravels

Unconfined/confined aquifer

Salinities range up to 20 000 mg/L. Low yields (< 5 L/sec) for stock water supply only.

Clay, sands and silts Confining layer

Tert

iary

Murray Group Fossiliferous limestone with sandy and marly interbeds

Confined aquifer Salinities range from ~1000 to 10 000 mg/L, yields 5 - 40 L/sec. Main source of groundwater irrigation supplies

Ettrick Formation Glauconitic, fossiliferous marl and calcareous clay

Confining layer

Renmark Group Carbonaceous sands and clays Confined aquifer Confined aquifer, not used extensively for irrigation due to low yields and discontinuous nature.

Cam

bra

in

Kanmantoo Group Metamorphosed sandstone, siltstone, greywacke

Fractured rock aquifer Poor aquifer, generally with high salinities and low yields.



GROUNDWATER FLOW AND SALINITY

Quaternary Aquifer

The water level elevation contours for the Quaternary aquifer in the Angas Bremer PWA are presented in Figure 3 and show groundwater flows generally towards Lake Alexandrina, in a north to south and north-west to south-east direction. In the vicinity of Langhorne Creek, a watertable mound has formed in the Quaternary aquifer in response to infiltration of river and floodwater from the Bremer River, causing groundwater to flow radially away from it to the east and south.

Groundwater salinities are generally lowest near streams where river recharge and occasional flooding occurs.

Figure 3. Groundwater flow and salinity distribution (2010) for the Quaternary aquifer in the Angas Bremer PWA



Confined Aquifer

The water level elevation contours for the confined limestone aquifer measured in March 2010 (at the end of the irrigation season) are presented in Figure 4 and show the general groundwater flow direction towards Lake Alexandrina in a south-easterly direction.

The salinity distribution in the confined limestone aquifer shows low-salinity groundwater limited to relatively narrow zones parallel to the Angas and Bremer Rivers, where salinity ranges from 1500 to 3000 mg/L. Towards the margins of the basin to the east and west, salinities can be as high as 10 000 mg/L. The small lens areas of low salinity (less than 1500 mg/L) near the lake are the result of managed aquifer recharge operations.

Figure 4. Groundwater flow and salinity distribution (2010) for the confined limestone aquifer in the Angas Bremer PWA



GROUNDWATER DEPENDENT ECOSYSTEMS Whilst groundwater dependent ecosystems (GDEs) have not been considered in the assessment of the status of this groundwater resource, it is important to note the presence and ecological characteristics of the GDEs in the Angas Bremer PWA. Water Allocation Plans must include an assessment of the water required by ecosystems; this includes water from both surface water and groundwater resources. Groundwater dependent ecosystems can be defined as ecosystems where groundwater provides all or part of the water quantity, chemistry or temperature requirements. It is generally considered that shallow watertables less than ten metres below the surface are more likely to support GDEs than deeper watertables. The exception to this is stygofauna (animals that inhabit water-filled spaces and pools below the ground) which can be found at greater depths.

Plants with a dependence on groundwater exist along the watercourses within the PWA and largely consist of River Red Gum (Eucalyptus camaldulensis). These trees may use groundwater from the shallow unconfined Quaternary aquifer, which is not widely used by licensees or stock and domestic users because of low yields and high salinities.

The Murray Group Limestone, which is developed for irrigation, is considered too deep below the surface to support any terrestrial vegetation or wetlands.



RAINFALL

The climate in the Angas Bremer PWA is dominated by hot, dry summers and cool, wet winters. The location of the Langhorne Creek rainfall station (24515) and rainfall isohyets are displayed in Figure 5.

Figure 5. Location of Langhorne Creek rainfall station (24515) and rainfall isohyets

The average annual rainfall for Langhorne Creek is 389 mm for the period 1889 to 2010. Figure 6 shows the annual rainfall recorded at Langhorne Creek (station 24515) in blue columns. The cumulative deviation is also plotted in orange and measures the difference between the actual measured rainfall and the long-term average rainfall on an annual basis. An upward trend in this line indicates above-average rainfall and conversely, a downward trend indicates below-average rainfall.



The Langhorne Creek rainfall station shows generally above-average rainfall from 1930 until 1956, which was followed by a period of mostly average rainfall until the extremely wet period in 1992. Since then, there has been below-average rainfall, particularly since 2006. Above-average rainfall was recorded in 2010.

Figure 6. Annual rainfall and cumulative deviation for mean monthly rainfall for Langhorne Creek

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1889 1899 1909 1919 1929 1939 1949 1959 1969 1979 1989 1999 2009

Cu

mu

lative Deviatio

n (m

m)

An

nu

al R

ain

fall

(mm

)

24515: Langhorne Creek

Trendline Cumulative Deviation Average 389 mm



GROUNDWATER USE

Metered groundwater extractions for licensed purposes (excluding stock and domestic use) in the Angas Bremer PWA totalled 5334 ML1 in the 2009–10 water-use year (Fig. 7), which represents a 17% decrease from the previous water-use year. The increase in extraction after 2005–06 was due to River Murray Prescribed Watercourse allocations sourced from Lake Alexandrina being unsuitable because of high salinity and access difficulties due to the receding shoreline. As a consequence, demand for groundwater increased significantly. The recent decrease in groundwater extractions reflects the recovery in the condition of the lake and increased access to River Murray water allocations via two new pipelines.

Figure 7. Historical licensed groundwater use for the Angas Bremer PWA

1 This figure may include use of groundwater allocations, managed aquifer recharge allocations and rollover allocations.

0

1000

2000

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7000

8000

2000-01 2001-02 2002-03 2003-04 2004-05 2005-06 2006-07 2007-08 2008-09 2009-10

Ext

ract

ion

(M

L)

Extraction limit = 6500 ML



MANAGED AQUIFER RECHARGE

Managed aquifer recharge commenced in the Angas Bremer PWA in the early 1980s. Until recently, most managed aquifer recharge occurred along the Bremer River, with fewer sites located adjacent to the Angas River and Mosquito Creek. The amount of surface water injected into the confined limestone aquifer from these sources varies significantly, depending on the availability of stream flow with salinities less than 1500 mg/L. The construction of two pipelines from the River Murray during the recent drought has increased the reliability and the volumes of water available for managed aquifer recharge.

Figure 8 shows that the volume of managed aquifer recharge water (displayed in blue) injected into the confined limestone aquifer during 2009–10 was 4803 ML, which represents a significant increase from previous years and a 123% increase from 2008–09.

Figure 8 also presents extraction volumes. For 2009–10, the overall net volume of water extracted from the confined limestone aquifer equates to only 531 ML (relative to 4240 ML net extraction in 2008–09). The volumes of water injected to the aquifer are likely to offset adverse impacts due to extraction.

Figure 8. Historical licensed extraction and managed aquifer recharge2 injection in the Angas Bremer PWA.

2 Source of data for managed aquifer recharge volumes from 2000–01 to 2002–03 is the Angas Bremer Water Management Committee. Data from 2003–04 onwards is sourced from DFW WILMA Database.

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7000

2000-01 2001-02 2002-03 2003-04 2004-05 2005-06 2006-07 2007-08 2008-09 2009-10

Vo

lum

e (M

L)

Extraction Managed aquifer recharge



GROUNDWATER OBSERVATION NETWORKS

WATER LEVEL NETWORK A groundwater monitoring network in the Angas Bremer PWA was established in the late 1960s in response to falling groundwater levels and rising groundwater salinity. There are currently 92 wells monitoring water levels in the Angas Bremer water level network, with 75 of these wells falling within the Angas Bremer PWA.

Water levels were monitored monthly until 2005, but since then have been monitored on a three monthly basis. The wells used to monitor water levels for all the aquifers in the Angas Bremer PWA are shown in Figure 9.

Figure 9. Location of water level observation wells in the Angas Bremer PWA



Table 2. Groundwater level observation wells per aquifer in the Angas Bremer PWA

Aquifer Number of wells

Quaternary 33

Murray Group Limestone 34

Murray Group Limestone and Renmark Group 4

Renmark Group 3

Murray Group Limestone, Renmark Group and Kanmantoo Group3

1

Total 75

A shallow (6 m deep) observation well network of 172 wells was established in 1997 by the Angas Bremer Water Management Committee to monitor shallow watertables beneath irrigated areas at risk of waterlogging. Of these, only 57 are currently recording water levels with the remainder now dry.

SALINITY NETWORK There are currently 81 wells monitoring salinity in the Angas Bremer network, with 73 of these wells falling within the Angas Bremer PWA, as displayed in Figure 10. The frequency of salinity monitoring has varied over the years; however typically occurs twice a year; once during autumn (i.e. following irrigation season) and once during spring (i.e. prior to irrigation season). Salinity observation wells are completed into all aquifers within the PWA, as detailed in Table 3.

Table 3. Groundwater salinity observation wells per aquifer in the Angas Bremer PWA

Aquifer Number of wells

Quaternary 32

Murray Group Limestone 33

Murray Group Limestone and Renmark Group 4

Renmark Group 3

Murray Group Limestone, Renmark Group and Kanmantoo Group

1

Total 73

3 Kanmantoo Group is also referred to as the Basement and is displayed in Figure 2 and Table 1.



Figure 10. Location of salinity observation wells in the Angas Bremer PWA



GROUNDWATER LEVEL TRENDS

CONFINED LIMESTONE AQUIFER (MURRAY GROUP) The groundwater level trends observed in the confined limestone aquifer are strongly controlled by groundwater extractions from this aquifer. As a result of the high connectivity in some areas, these extractions also influence trends in the overlying and underlying aquifers. Figure 11 depicts trends from throughout the Angas Bremer PWA that all show seasonal fluctuations to varying degrees and a decreasing or stable trend prior to the significant reduction in groundwater extraction in the early 1990s.

The reductions in groundwater extraction were in response to the increased availability of River Murray water allocations sourced from Lake Alexandrina. This reduction in groundwater extractions resulted in a substantial recovery in groundwater levels, which continued until the 2006 drought when extractions increased due to the decreasing availability of River Murray water from Lake Alexandrina (due to access and salinity problems).

Drawdowns in the confined limestone aquifer reached a peak during early 2009 to early 20104 after which flood waters flowing into the lake and two new pipelines from the River Murray reduced demand on groundwater. The reduced demand on the groundwater resources coupled with significant volumes managed aquifer recharge water injected to the aquifer produced a recovery in water levels during 2010.

Locally, managed aquifer recharge activities can enhance the recovery of groundwater levels during winter and spring as shown by the increased amplitude in well FRL064 in the early 1990s.

Figure 11. Groundwater level trends for the confined limestone aquifer in the Angas Bremer PWA

4 February 2009 (STY110), November 2009 (FRL004) and March 2010 (wells BRM007 and FRL064)

-5

0

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1970 1975 1980 1985 1990 1995 2000 2005 2010

Wat

er le

vel e

leva

tio

n (

m A

HD

)

BRM007 FRL004 FRL064 STY110

Latest readings October 2010



QUATERNARY AQUIFER A variety of monitoring trends for the unconfined Quaternary aquifer can be seen in Figure 12, with the impact of the very wet 1992–93 season visible in most observation wells. The cumulative deviation from Figure 6 is plotted in orange and shows that below-average rainfall was experienced following the 1992 high rainfall event. Observation well FRL224 displays a reasonably close relationship with rainfall, with BRM222, located close the lakeshore, showing a more subdued response.

Observation well FRL216 is located adjacent to the Bremer River and displays marked seasonal variations which indicate recharge to the shallow unconfined aquifer by infiltration of streamflow. These seasonal variations may also indicate some degree of connection with the underlying limestone aquifer.

Well FRL226 is located on the eastern end of Mosquito Creek (Fig. 9) and shows an almost continuous decline of about 3 m since 1976. The lack of seasonal variation since about 1997 is probably the result of a lack of river water flows that are of sufficient magnitude to reach the end of the Mosquito Creek system and provide recharge to the watertable in that area.

Figure 12. Groundwater level trends for the Quaternary aquifer in the Angas Bremer PWA

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1975 1980 1985 1990 1995 2000 2005 2010

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lative de

viation

(mm

) W

ater

leve

l ele

vati

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(m

AH

D)

BRM222 BRM237 FRL216 FRL224 FRL226 Cumulative deviation

Latest readings June 2010



RENMARK GROUP AQUIFER Groundwater level trends in the Renmark Group confined aquifer (from which no known extractions occur) are almost identical to those in the overlying limestone aquifer (Fig. 13), which suggests good connectivity between the aquifers and a lack of a continuous impermeable confining layer.

Figure 13. Groundwater level trends for the Renmark Group aquifer in the Angas Bremer PWA

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4

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12

1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Wat

er le

vel e

leva

tio

n (

m A

HD

)

BRM156 BRM159 FRL138 STY108

Latest readings June 2010



GROUNDWATER SALINITY TRENDS

CONFINED LIMESTONE AQUIFER (MURRAY GROUP) Increases in salinity in the confined limestone aquifer have been identified as the main threat to the long term sustainability of irrigation in the Angas Bremer PWA. Downward leakage from the overlying brackish to saline Quaternary aquifer is the predominant cause of such salinity increases and is driven by the head difference between the two aquifers. The head difference and downward leakage are higher during periods of high extraction both on a regional scale and locally where large drawdowns in an individual irrigation well may induce significant leakage. The relationship between extraction and salinity is well illustrated in Figure 14, which shows the salinity in observation well BRM030 in blue on the left axis and the extraction for the whole of the Angas Bremer PWA in red on the right axis.

Figure 14. Relationship between extraction and salinity in the Angas Bremer PWA

Figure 15 presents trends from a number of observation wells throughout the Angas Bremer PWA. Whilst the trends may be affected by a number of processes, such as nearby managed aquifer recharge activity in wells, most show a decrease since the period of high extraction before the early 1990s and more significantly, an increase in salinity since the 2006 drought when extractions increased significantly.

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1985 1988 1990 1993 1996 1999 2002 2005 2008

Extraction

(ML/y) Sa

linit

y (

mg/

L)

Groundwater use BRM030



Figure 15. Salinity trends for the confined limestone aquifer in the Angas Bremer PWA

Most observation wells are showing a reduction in salinity during 2009–10 in response to reduced extractions and the injection of fresh water (salinity less than 1500 mg/L) through managed aquifer recharge.

Salinity trends for observation wells along the eastern and western margins of the PWA where salinities are higher are shown in Figure 16. These trends are not influenced by managed aquifer recharge and four of the five wells are displaying a gradual continuous rise in salinity averaging 31.8 mg/L/y since monitoring began in 1985. This increase is most probably the result of the slow lateral movement of locally more-saline groundwater down gradient toward the lake. Downward leakage from the Quaternary aquifer is not likely to be the cause, as the head difference indicates upward leakage from the limestone aquifer at some wells and where downward leakage is indicated, the salinities of the two aquifers are very similar.

Figure 16. Groundwater salinity trends for the confined limestone aquifer in the margins of the

Angas Bremer PWA

1000

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1985 1990 1995 2000 2005 2010

Sa

lin

ity

(m

g/L

)

BRM037 BRM153 BRM154 FRL004 FRL141 STY110

Latest readings Oct 2010

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1985 1990 1995 2000 2005 2010

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ity

(m

g/L)

BRM038 BRM157 BRM160 FRL067 FRL082




QUATERNARY AQUIFER Groundwater salinity trends in the Quaternary aquifer are quite variable as shown in Figure 17. Long-term increases since 1975 at rates of 26 and 95 mg/L/y have been observed in wells FRL216 and FRL226 respectively, whilst an even higher rate of 144 mg/L/y has been recorded for well BRM247 since 1993. The cause of these increases is most likely the process of evapotranspiration from the relatively shallow watertable, which removes water but leaves salt behind. Well FRL234 shows a decrease in salinity that may be the result of low-salinity irrigation drainage water recharging the aquifer.

Figure 17. Groundwater salinity trends for the Quaternary aquifer in the Angas Bremer PWA

0

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1975 1980 1985 1990 1995 2000 2005 2010

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ity

(m

g/L)

FRL226 FRL234 FRL216 BRM247 BRM244




RENMARK GROUP AQUIFER Salinity trends in the Renmark Group aquifer as shown in Figure 18 are relatively stable. The sudden decline in salinity in 1993 for well BRM159, which is located adjacent to the Bremer River, was possibly caused by the ingress of flood waters into the well, or via managed aquifer recharge.

Figure 18. Groundwater salinity trends for the Renmark Group aquifer in the Angas Bremer PWA

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1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Salin

ity

(m

g/L)

BRM159 STY108 FRL138


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